1. Field of the Invention
This invention relates to the convex bending of plates of thermoplastic materials and, in particular, of glass plates, available on a horizontal or roughly horizontal support, such as a delivery conveyor, that convexing taking place by application of the plates, brought to their softening temperature, against an upper mold and then possibly by sinking of said plates on a lower mold after being deposited or dropped on said lower mold, that lower mold generally consisting of a ring with curved section, open in the center, also called a skeleton.
2. Background of the Prior Art
The upper mold is a mold of the type described in French patent documents, one having been published under No. 2,085,464 and another filed under No. 8604962; that is, it is connected to a suction box and the suction of the box is exerted on the periphery of the mold, thus bringing the plates, notably, of glass, to be applied against said mold. The upper mold can also be of another type described in European patent publication No. 3,391, namely, it can consist of a unit drilled with holes, through which a vacuum is exerted.
The plates to be bent into convex shape, taking as an example only glass plates, knowing that they can nevertheless be of any thermoplastic material, are taken up by the upper mold thanks to the suction exerted on the periphery (patent document FR No. 2,085,464 or FR-A No. 8604962) or across the upper mold (European patent 3,391), or thanks to a stream of gas directed upward, against the glass sheet, so as to lift it (European patent 5,306).
The positioning of the glass plates under the upper mold has to be very precise, particularly, when said upper mold is curved.
When the glass plates arrive on a conveyor, their stop position under the upper mold can be very well controlled, in the longitudinal direction of passage of the conveyor, thanks to lugs, e.g., of the type described in the French patent document filed under No. 8513801.
On the other hand, the orientation and position in the directions crosswise to the direction of passage are more difficult to control. It is, of course, possible, as is done at the present time, to define the position of the glass plates far up the line, before they enter the reheating furnace preceding the convexing station. After a number of trials, the position to be given the glass plates can be defined above the furnace, so that they will be stopped in good position under the upper convexing mold. But that implies trial and error, necessitates frequent corrections, does not afford extreme precision and does not protect the plates from accidental deviations during their transport.
It is also possible to provide recentering devices for the glass plates under the upper mold, but that causes said plates to rub on their support, particularly, the rollers of the conveyor, which creates the risk of damaging the surface of the glass, and if the stresses applied on the edges of the plates in order to recenter them are really substantial, that can also damage the edges of the plates.